Bearing cages for rolling-element bearings are generally comprised of two lateral rings disposed in an axial spacing (i.e. in the direction of a bearing rotational axis) and a plurality of bridges connecting these lateral rings and disposed one-behind-another in a circumferential direction of the bearing cage, which bridges, in pairs, form pockets for receiving rolling elements. Using the provided pockets, a bearing cage holds the rolling elements spaced with respect to each other and prevents a direct contact of adjacent rolling elements, which typically reduces friction and thus heat generation. In addition, it ensures a uniform distribution of the rolling elements over an entire circumference of the cage or rolling-element bearing, and thus makes possible a uniform load distribution as well as a quiet and uniform operation.
In operation, bearing cages are highly mechanically stressed by friction-, tearing-, and inertia forces. Chemical influences due to certain additives and substances can also potentially result. Design and material selection are therefore altogether of decisive importance for functional efficiency of the cage as well as for operational reliability of the bearing.
Rolling-element bearing cages typically comprise pressed cages and solid cages. Pressed cages for rolling-element bearings are usually produced from steel plate, in some cases also from brass plate. Solid cages for rolling-element bearings can, for example, be manufactured from brass, steel, aluminum, polymers, or phenolic resin.
Plastic solid cages, which are often produced using injection-molding methods, are characterized by a favorable combination of strength and elasticity. Good sliding properties of plastic on lubricated steel surfaces and a low roughness of the cage surfaces at contact points with rolling elements have the consequence of a low cage friction, a correspondingly low heat generation in the bearing, and scarcely measurable wear. Due to the low material density, forces from the inertia of the cage also remain small. Thanks to good emergency running properties of plastic cages, even with complete failure of the lubrication the bearing remains functional for some time without resulting in seizing of the bearing or other consequential damage.
Plastics for commonly injected rolling-element bearing cages can be, for example, polyamide 66, polyamide 46, polyetheretherketone (PEEK), phenolic resin, or also another polymer material.
Large rolling-element bearings, such as, e.g., tapered roller bearings, starting from a bearing diameter of, for example, >300 mm, can usually be only be assembled with a steel-bolt cage or with a steel cage separately. The manufacture of both cage types is very expensive. However, with alternative plastic cages the problem is that they can be manufactured only with difficulty or not in the required quality for medium-to-large bearing diameters, for example starting from a diameter of approximately 300 mm. This is due to, among other things, the thermal expansion coefficient of plastic, which is substantially greater than that of steel, so that in a purely plastic cage, when heat is generated a clamping effect of the rolling elements can result. Due to the increased thermal expansion coefficient, shoulder-guiding of a plastic cage cannot be ensured. Furthermore, due to the increased thermal expansion coefficient the dimensional accuracy with respect to the bearing cage diameter diminishes. Compared to metal, such as, for example, steel, the strength of plastic in the radial direction is also significantly limited. In addition, complicated injection-molding tools would be needed to manufacture plastic cages with such a large diameter, which in turn would lead to unacceptably high manufacturing costs.